The invention pertains to a cutting bit, including the cutting insert therefor, wherein the bit is useful for cutting through various earth strata. Specifically, the invention pertains to a roof drill bit for drilling bore holes in an underground mine.
The expansion of an underground mine, such as for example, a coal mine, requires digging a tunnel. Initially, this tunnel has an unsupported roof. Because the roof is not supported, there is an increased chance for a mine cave-in which, of course, adds to the dangers and safety hazards of underground coal mining. Furthermore, an unsupported roof is susceptible to rock and debris falling from the roof. This rock and debris can injure workers as well as creating hazardous clutter on the floor of the tunnel.
In order to support and stabilize the roof in an established area of an underground tunnel, bore holes are drilled in the roof. The apparatus used to drill these holes comprises a drill with a long shaft, i.e., drill steel, attached to the drill. A bit is detachably mounted to the drill steel at the distal end thereof. The bit is then pressed against the roof, and drilling apparatus operated so as to drill a bore hole in the roof. The bore holes extend between two feet to greater than twenty feet into the roof. At this point in the roof bolting operation, there is no overhead protection for the operator.
These bore holes are filled with resin and roof bolts are affixed within the bore holes. A roof support, such as roof panels is then attached to the roof bolts. The end result is a roof which is supported, and hence, is of much greater stability than the unsupported roof. This reduces the safety hazards associated with underground mineral mining. The roof bolting process is considered to be an essential underground mining activity.
Roof bolting accounts for the largest number of lost-time injuries in underground mining. During the roof bolting process, the roof is unsupported so that it does not have optimum stability. Furthermore, the roof bolting process exerts stresses on the roof so as to further increase the safety hazards during the roof bolting process. Thus, an increase in the speed at which the bore holes can be drilled contributes to the overall speed and efficiency of the roof bolting process.
The speed of drilling the bore holes is dependent upon the sharpness and useful life of the roof drill bit. A sharp roof drill bit results is faster penetration and drilling. A sharp roof drill bit also does not require as much force to be applied to drill the bore hole. A roof drill bit that is dull requires a great amount of force to be exerted on the drill bit which can lead to bending the drill steel. A roof drill bit that stays sharp provides for good speed and penetration in the drilling operation.
When a roof drill bit becomes dull, i.e., has worn past its useful life, the drill bit must be changed before further drilling can be undertaken. In order to change the roof drill bit, the drilling must be stopped, the drill steel and drill bit removed from the bore hole in process, the worn drill bit removed from the drill steel, and the new roof drill bit attached to the drill steel. This operation takes time away from the productive drilling process and decreases the overall efficiency of the roof bolting process.
The necessity to change the roof drill bit also keeps the operator down in the section of the mine that has an unsupported roof. The longer that the operator stays in the section of the mine with an unsupported roof, the greater the chances are that there will be injury due to the unsupported mine roof. The advantages of a longer drill bit life are very apparent.
Sometimes the roof drill bit may encounter a portion of the roof strata which will cause great stresses to be exerted on the roof drill bit as well as the connection between the drill bit and the drill steel. These stresses can cause the drill bit to fail or the connection between the drill bit and the drill steel to fail. Either one of these conditions is an undesirable consequence which can lead to reduced efficiency for the roof bolting process. The advantages of a strong drill bit and a connection between the drill bit and drill steel of good integrity are apparent.
In the past, cemented tungsten carbide has been the most popular material to use for the cutting insert in the roof drill bit. Cemented tungsten carbide has been recognized for many years as a hard material that is very suitable for application as a cutting insert in a roof drill bit. However, cemented tungsten carbide cutting inserts are typically capable of drilling only a limited number of bore holes, such as, for example, one or two four foot holes or even less (sometimes to a depth of only a few inches) depending on the earth (e.g. rock) strata being drilled, before it is necessary to use another cemented tungsten carbide cutting insert.
Typically, the cutting inserts are resharpened at a location remote from the location of the roof bolting process. Thus, in order to resharpen the cutting insert, the drill bit must be removed from the bore hole, the drill bit removed from the drill steel, and a new drill bit mounted to the drill steel. The cutting inserts can be resharpened so as to be able to drill again. It would be desirable to make a cutting insert that could be used more than once without the need to be resharpened.
Although cemented tungsten carbide has worked in a satisfactory fashion for many years, it would be desirable to make the cutting insert from a material that would retain its sharpness longer than cemented tungsten carbide. It would also be desirable to make the cutting insert from a material that would have a useful life longer than that of cemented tungsten carbide.
Heretofore, others have used polycrystalline diamond (PCD) compacts as cutting inserts in some applications. For example, U.S. Pat. No. 4,928,777 to Shirley-Fisher shows a polycrystalline cutting insert useful in a rotary drill bit such as used in the petroleum industry. U.S. Pat. No. 4,373,593 to Phaal et al. shows a polycrystalline diamond cutting insert for a rotary drill bit. Other patents which show the use of polycrystalline cutting inserts in rotary drill bits, which are typically used in the oil drilling industry, are U.S. Pat. Nos. 4,989,578 to Lebourg, 4,911,254 to Keith, 4,529,048 to Hall, 4,694,918 to Hall, and 4,811,801 to Salesky et al.
Heretofore, others have used a cutting insert which includes polycrystalline diamond as a cutting insert for a roof drill bit. U.S. Pat. No. 4,627,503 to Horton shows a roof drill bit that uses one laminate PCD cutting insert of a conventional shape.
The Brady's Mining and Construction Supply Co. of St. Louis, Miss. has introduced what they call a "high density ceramic" roof bit. The bit comprises an elongate shank integral with a pair of larger diameter lobes at the forward end thereof. A flat surface has been machined in these lobes whereby the flat surface receives a semicircular cutting insert. The cutting insert is made from a PCD composite. The bit attaches to the end of a drill steel via a special adaptor and a plurality of roll pins. Although the Brady bit presents the use of a PCD composite as a cutting insert in a roof drill bit, there are a number of disadvantages incumbent with the Brady bit that would be highly desirable to overcome.
The Brady bit cutting insert is of a semi-circular shape. Because of the orientation of the cutting insert on the roof drill bit, much of the semi-circular cutting edge does not actually participate in the drilling. The presence of this portion of the PCD composite is unnecessary. Due to the relatively expensive nature of PCD composite cutting inserts, it would be highly desirable to provide a cutting bit useful for cutting earth strata, such as a roof drill bit, with a PCD composite cutting insert that does not have a wasted length of the cutting edge.
PCD composite cutting inserts are typically made from a circular blank. In the case of the Brady bit, the circular blank is cut in half to make two semi-circular cutting inserts. PCD composite blanks are relatively expensive. It would be highly desirable to provide a cutting bit useful for cutting earth strata, such as a roof drill bit, that has a PCD composite cutting insert of such a shape so as to make more efficient use of the circular PCD blank from which the cutting insert is made.
The Brady bit body has a sudden increase in diameter at the junction of the larger diameter lobes and the integral shank. Because of this sudden increase in diameter, there is the potential for the bit to fail under torsional forces at this juncture. It would be highly desirable to provide a cutting bit useful for cutting earth strata, such as a roof drill bit, that uses a PCD composite that does not have a propensity to failure under torsional forces, especially due to a difference in diameter along the length of the bit body.
A cutting insert made of PCD must not reach an elevated temperature, such as 1200.degree. F. for over a certain duration, such as two minutes, or it will become brittle and its usefulness meaningfully reduced. The Brady bit body contains a water channel in the shank portion of the bit body. However, experience shows that water does not adequately reach the cutting insert because of turbulence caused by the water impinging upon the greater diameter lobes of the bit body. It would be highly desirable to provide a cutting bit useful for cutting earth strata, such as a roof drill bit, that provides a uniform and consistent water supply to the PCD composite cutting insert so that it will not fail due to temperature-related causes.
Because of the use of the special adapter and roll pins, the Brady bit requires a relatively long time to change bits in the field. It would be highly desirable to provide a roof drill bit that does not require a relatively long time to change bits in the field. The presence of a special adapter presents one more piece of structure to have the potential to fail in the field. It would be highly desirable to provide a cutting bit useful for cutting earth strata, such as a roof drill bit, that does not require a special adapter to attach the bit body to the drill steel.
The PCD composite cutting insert of the Brady bit lies in a flat machined out surface. The flat surface of the PCD composite cutting insert is brazed to the flat surface of the machined surface in the body. When the bit is placed under load in drilling, there is only one surface for the cutting insert to load against in the Brady bit. Such a circumstance can lead to shear stress failure of the PCD composite cutting insert, i.e., shear stresses catastrophically remove the cutting insert from the bit body. It would be highly desirable to provide a cutting bit useful for cutting earth strata, such as a roof drill bit, that is able to distribute the loading forces over more than one surfaces so as to reduce the potential for shear stress failure.
The semi-circular cutting insert of the Brady bit cannot be reused once it has passed its useful life. Because of the expense associated with PCD composite cutting inserts, it would be highly desirable to provide a cutting bit useful for cutting earth strata, such as a roof drill bit, that has a cutting insert which can be used more than once.
The orientation of the cutting inserts in the Brady bit are such that the cutting edges drill the entire transverse cross-section of the bore hole. It is known that drilling may proceed faster if the center of the bore hole is not in contact with the cutting inserts. This is the case for conventional two-prong bits that use cemented tungsten carbide cutting inserts. It would be desirable to provide a cutting bit useful for cutting earth strata, such as a roof drill bit, with a PCD composite cutting insert that does not drill across the entire transverse dimension of the bore hole.